This invention relates to providing a turbine engine with rotors or turbine discs which are made of ceramic materials such as silicon nitride or silicon carbide. In particular, the invention relates to the high thermal stress connection between the metallic shaft of a turbine engine and a ceramic turbine rotor having a shortened or stub shaft.
It is known that to provide a turbine engine, such as a gas turbine or a turbocharger, with ceramic rotors or turbine discs offers substantial advantages in the form of increased temperature stability and reduced moment of inertia. In terms of building the mechanisms and integrating the assemblies of the overall engine, however, it is better to make the turbine shaft out of metallic materials. Connecting the ceramic turbine discs (or rotors) to metallic shafts has become quite a costly and problematic task due, primarily, to the brittleness of the ceramic materials and the markedly different coefficients of thermal expansion of metallic material and ceramic material.
There are presently known arrangements in which the turbine discs are connected to the shaft by means of tension bolts through a central bore of the disc. The face of the disc in contact with the shaft is then generally executed in the form of a Hirth serration. The surfaces of contact between ceramic and metal call for heavy manufacturing outlay, since point loads must be avoided. The bolt needs to be elaborately designed to equalize differential heat expansion of disc and shaft. Besides, in the known types of connection, the critically stressed disc needs to be provided with a bore. But such discs provided with through holes incur service stresses about twice as high as those in unbored discs. It has likewise been proposed in United States patent Application Ser. No. 40,121, dated May 18, l979, that a base portion belonging to a ceramic turbine blade be soldered directly to a metal shaft part enclosing the stub. Owing to the great differences in thermal expansion between the parts to be connected, high shrinkage stresses arise at the soldered joints during cooling after the soldering operation. These shrinkage stresses give rise to high compressions in the ceramic base, which in principle the ceramic material is well able to withstand. At the transition from the soldered joint to the exposed blade, however, stress concentrations occur, with high tensions that the ceramic material will not tolerate. Under service conditions with cyclic temperature stress, these compressions can be cut down by plastic deformation. But upon further thermal stress, high tensions will be generated at the junction, and may lead to failure of the soldered joint or of the ceramic base.
It is therefore an object of the present invention to provide a connection between a metallic shaft and a ceramic disc which eliminates the problems caused by the great differential in the coefficients of thermal expansion between ceramic material and metallic material.
It is a further object of the invention to provide a means for connecting a metallic shaft to a ceramic disc wherein the problems of a temperature stressed ceramic disc normally associated with a soldered connection between such components is eliminated.